The present invention relates generally to the field of digital communications. More specifically, the present invention relates to circuits and processes for balancing in-phase and quadrature phase components of received complex communication signals.
Digital communication systems include a transmitter and a receiver. The transmitter and receiver each typically include a digital section and an analog section. The transmitter digital section converts data to be communicated into a digital form which is more suitable for conveyance over a communication link. The analog section translates this converted digital data into an analog form which is often upconverted prior to direct application to the communication link. The receiver analog section obtains an analog communication signal from the communication link and typically downconverts the analog signal into an analog form which is suitable for digitizing, then digitizes the downconverted signal. The receiver digital section then processes the digitized signal to extract the conveyed data. Typical digital communication systems use phase quadrature, complex signals to convey data through the digital and analog sections of the transmitter and receiver.
In-phase to quadrature phase (I-Q) imbalance has long plagued digital communication systems. I-Q imbalance occurs when the quadrature phase signal components which have been modulated to convey data are not perfectly in quadrature (i.e., separated in phase by 90xc2x0) or are otherwise processed unequally, such as by actually applying differing gain to in-phase and quadrature signals when equal gain is desired. I-Q imbalances typically occur at least in the analog sections of the communication system, particularly in connection with upconversion and downconversion. Moreover, some amount of I-Q imbalance is inevitable because it results from the use of analog components, such as RF mixers, which fail to achieve absolute perfection in the performance of their functions. A consequence of I-Q imbalance is distortion in the communication signal, which impairs the ability of the receiver to correctly extract the data conveyed by the communication signal.
One solution to I-Q imbalance includes the use of accurate analog components in the analog sections of the transmitter and receiver. For example, a typical digital communication receiver which demodulates 16-QAM data may specify the use of components which achieve around one dB in gain imbalance and 1-2 degrees in phase imbalance when adaptive equalization is being used. Such components, while slightly expensive, are well within mass market manufacturing capabilities. Unfortunately, as digital communications operate at higher orders of modulation (e.g., 64-QAM), susceptibility to I-Q imbalance distortion becomes more pronounced, requiring analog components which are too expensive to be practical in mass market equipment.
Another solution to I-Q imbalance is to use adaptive circuits which may be adjusted as needed to compensate for I-Q imbalance. Improvements may result when the adaptive circuits are analog in nature. However, adaptive analog circuits tend to be undesirably expensive, and being analog circuits themselves, only a limited amount of I-Q balancing results.
Digital adaptive equalizers in the digital sections of receivers provide a number of beneficial results. In addition to reducing I-Q imbalance, they often compensate for other types of distortion, such as intersymbol interference (ISI). Adaptive equalizers are typically digital filters whose taps are varied in a feedback loop to maximize data quality. An adaptive equalizer may be placed inside a carrier tracking loop, where it operates on a baseband signal, or outside the carrier tracking loop, where it operates on an intermediate frequency (IF) signal.
If an adaptive equalizer is located outside the carrier tracking loop where it processes an IF signal, it can compensate only for I-Q imbalance induced in the receiver. Moreover, its ability to compensate for other types of distortion, such as ISI, is diminished compared to locating the adaptive equalizer within a carrier tracking loop.
Improved performance almost always results from locating the adaptive equalizer within the carrier tracking loop so that it operates on the baseband signal, and making that carrier tracking loop a data or decision directed feedback loop. A data or decision directed feedback loop uses the data conveyed through the communication link and extracted by the receiver to generate an error signal that closes the feedback loop. Such baseband adaptive equalizers adequately compensate for I-Q imbalance induced both in the transmitter and receiver and for other types of distortion, such as ISI. Unfortunately, such baseband adaptive equalizers require a sufficiently low amount of I-Q imbalance without equalization that valid data may be recovered for use in operating the data directed feedback loop. This low amount of I-Q imbalance without equalization requirement still dictates the use of highly accurate and undesirably expensive analog circuits, particularly when higher orders of modulation are present.
An adaptive equalizer may be effective at compensating for I-Q imbalances and other distortions without requiring valid data to be extracted from the communication link when an a priori known training sequence is used to train the adaptive equalizer. However, the use of a training sequence is undesirable because it requires additional overhead that causes a corresponding reduction in the data-conveying capacity of the communication system, and it complicates the design by forcing the transmitter and receiver to accommodate a training sequence class of data that differs from other data.
It is an advantage of the present invention that an improved digital communication receiver having a digital, intermediate frequency (IF), in-phase to quadrature phase (I-Q), balancer is provided.
Another advantage of the present invention is that I-Q balancing is performed without requiring the extraction of valid data or adaptation to a known training sequence.
Another advantage of the present invention is that I-Q balancing is performed in a manner which permits the use of readily available, inexpensive, commercial grade analog components.
Another advantage of the present invention is that a given grade of analog components may be used to communicate at higher modulation orders.
These and other advantages are realized in one form by an improved digital communication receiver. The digital communication receiver includes an analog downconversion section which provides a complex, digitized, intermediate frequency (IF) communication signal exhibiting an in-phase to quadrature phase (I-Q) imbalance. An I-Q balancer having a signal input adapted to receive the IF communication signal has an output providing a locally balanced IF communication signal. A carrier tracking loop has an input adapted to receive the locally balanced IF communication signal. The carrier tracking loop converts the locally balanced IF communication signal into a baseband communication signal, and the carrier tracking loop has an equalizer which equalizes the baseband communication signal.